This invention relates to a method and an apparatus for distilling (meth)acrylic acid and/or (meth)acrylic ester in a distillation tower.
Distillation is an operation in which a raw liquid composed of two or more liquids in a mixed manner each having a different boiling point is heated for vaporization and the gas (or vapor) mainly composed of a component having a lower boiling point is condensed to separate the components of the mixture of liquids. In the case where the relative volatility between the components is great, the separation of the components is easy. However, there comes a problem that, in a normal state, the components cannot be separated with a desired concentration by merely vaporizing and condensing part of the mixture of liquids. Accordingly, it is a general practice in the art of distillation that part of the condensed liquid is returned (refluxed) from the top of the distillation tower while passing a tray or a surface of a packing provided in the tower to render the gas rising up through the distillation tower into contact with the reflux liquid returning from the top of the tower in an adequate manner so as to withdraw the component having a lower boiling point through the top of the distillation tower while withdrawing the component having a higher boiling point through the bottom of the distillation tower. The efficiency (distillation performance) of the distillation tower can be theoretically calculated according to the specification of the tower, the flow rate of the liquid and the gas to be supplied through the tower, and the property thereof.
If, however, the distributing ability of a liquid to be supplied into the distillation tower (hereinafter, also referred to as "supply liquid") such as a raw liquid and a reflux liquid is low, the supply liquid unevenly flows through the distillation tower. Accordingly, it takes a plurality of trays when the apparatus/method adopts the tray tower, or a packing with a thickness of several ten millimeters is required when the apparatus/method adopts the packed tower system to evenly distribute the liquid.
Namely, in the conventional method and apparatus, a certain level of distillation performance that would have been obtained according to the theory could not be obtained occasionally. Especially, if the diameter of the distillation tower is large, or the surface tension of the supply liquid is great, or the viscosity of the supply liquid is high, or the supply liquid has a property that the liquid is easily separated into two liquid phases of oil phase and water phase, a problem has been pointed out that the liquid is liable to unevenly pass through the distillation tower, which would lower the distillation efficiency. Especially, the prior art apparatus/method has suffered from a problem that the distillation efficiency is gradually lowered in the case of distilling (meth)acrylic acid and/or (meth)acrylic ester.
FIG. 1 is a cross sectional view exemplifying a tray tower 10 which is generally used in the prior art when distilling (meth)acrylic acid and (meth)acrylic ester (hereinafter, also simply referred to as "(meth)acrylic acid" as a general term for (meth)acrylic acid and (meth)acrylic ester). In this embodiment, described is a case where a raw liquid in which (meth)acrylic acid is contained as a main component and acetic acid is contained as an impurity is used for distillation. The raw liquid is supplied into the distillation tower 10 through a line L.sub.1. Acetic acid having a lower boiling point rises up through the distillation tower 10 while being vaporized by heating, and is introduced into condensing means 20 via the top of the distillation tower 10 through a line L.sub.2. After condensation by the condensing means 20, the condensed liquid is drawn out through a line L.sub.3.
On the other hand, (meth)acrylic acid, having a higher boiling point, is drawn out from the bottom of the distillation tower 10 to a next step through a line L.sub.4. Part of a refined liquid obtained by the above distillation is returned to the distillation tower 10 via a reboiler 30 through a line L.sub.5. Since the condensed liquid by the condenser 20 contains (meth)acrylic acid as well as acetic acid (impurity), part of the condensed liquid is returned into the distillation tower 10 through a line L.sub.6 as a reflux liquid to improve collecting efficiency of (meth)acrylic acid. After subject to gas-liquid contact in the distillation tower 10, (meth)acrylic acid in the reflux liquid flows down onto the bottom of the distillation tower 10 and is drawn out to the next step.
Since (meth)acrylic acid is a compound liable to be polymerized quite easily, it is a general practice to add a polymerization inhibitor such as hydroquinone and phenothiazine to the raw liquid and the reflux liquid. Also, there has been adopted a method for supplying oxygen through a line L.sub.7 to the bottom portion of the distillation tower 10 to suppress polymerization.
Despite of the above countermeasures, however, polymerization of (meth)acrylic acid cannot be completely prevented. Containment of polymerized matters in the distillation tower 10 lowers purification performance in the distillation tower 10. Accordingly, what frequently happened was to suspend operation of the distillation tower 10 for removing the polymerized matters accumulated in the distillation tower 10.